In recent years, with the growing concern over environmental problems, the demand for good fuel economy for automobiles has been becoming stronger, and there is also a demand for excellent fuel economy for rubber compositions used for automobile tires. As polymer compositions for automobile tires, a rubber composition containing a conjugated diene polymer such as polybutadiene or butadiene-styrene copolymer and a reinforcing agent is used.
For example, JP-B-1-29802 (JP-B denotes a Japanese examined patent application publication) proposes a styrene-butadiene copolymer of a high vinyl bond, and a polymer composition using the copolymer.